JP-H11-322163-A2 describes a problem in paragraph 0295 and FIG. 63 that the height of a stack of folded sheet bundles is much higher only at a side of the folded edge if the folded sheet bundles are stacked with each folded edge overlapping one another because of a spring effect each possesses even if each bundle is folded strongly. In the situation of stacked folded sheet bundles, the open sides of the stack, that is, the side opposite of the folded edges, do not have such a high height. If extra sheet bundles are continually added on the stack, the stack eventually collapses towards the side of the open ends.
JP-H11-322163-A2 further describes a stay 106a to avoid such an occasion of stack collapse. The stay 106a is almost the same height as the height of a stack of predetermined number of sheet bundles with their folded edges overlapping each other. The stay 106a is set under the open end side of the stack. However, the stay 106a is not sufficient enough to support various kinds of sheet bundles because the individual height of the sheet bundles changes depending on such factors as temperature and humidity.
JP-H11-322163-A2 yet describes a proposed solution to avoid such a voluminous stacking in paragraph 0293 and FIG. 62. The proposed solution is to stack the sheet bundles with shifting each folded edge of a bundle off from the folded edge of other bundles to an open end side of a sheet bundle below, individually. However, this proposed solution raises another problem. Specifically, an increasing number of sheet bundles undesirably increases the size of the footprint of the stack.
Moreover, JP-2003-261256-A2 describes controlling a moving distance of a sheet stopper mechanism moving in a horizontal direction on a basis of the height of a stack of sheet bundles on an inclined sheet stacker to increase a load capacity.
But the control does not work well before the stack exceeds a predetermined height. In other words, the stack of sheet bundles tends to be unstable when the stack is higher than the predetermined height. The stack of sheet bundles also tends to be unstable after stacking many sheet bundles because sheet bundles stop at the horizontal floor where the sheet stopper moves around.